Angiogenesis is the fundamental process by which new blood vessels are formed and is essential to a variety of normal body activities (such as reproduction, development and wound repair). Although the process is not completely understood, it is believed to involve a complex interplay of molecules which both stimulate and inhibit the growth of endothelial cells, the primary cells of the capillary blood vessels. Under normal conditions, these molecules appear to maintain the microvasculature in a quiescent state (i.e. one of no capillary growth) for prolonged periods which may last for as long as weeks or in some cases, decades. When necessary however (such as during wound repair), these same cells can undergo rapid proliferation and turnover within a 5 day period. (Folkman, J. and Shing, Y., The Journal of Biological Chemistry, 267(16): 10931-10934, and Folkman, J. and Klagsbrun, M., Science, 235: 442-447 (1987)).
Although angiogenesis is a highly regulated process under normal conditions, many diseases (characterized as "angiogenic diseases") are driven by persistent unregulated angiogenesis. Otherwise stated, unregulated angiogenesis may either cause a particular disease directly or exascerbate an existing pathological condition. For example, ocular neovacularization has been implicated as the most common cause of blindness and dominates approximately 20 eye diseases. In certain existing conditions such as arthritis, newly formed capillary blood vessels invade the joints and destroy cartilage. In diabetes, new capillaries formed in the retina invade the vitreous, bleed, and cause blindness. Growth and metastasis of solid tumors are also angiogenesis-dependent (Folkman, J., Cancer Research, 46: 467-473 (1986), Folkman, J., Journal of the National Cancer Institute, 82: 4-6 (1989)). It has been shown for example that tumors which enlarge to greater than 2 mm, must obtain their own blood supply and do so by inducing the growth of new capillary blood vessels. Once these new blood vessels become embedded in the tumor, they provide a means for tumor cells to enter the circulation and metastasize to distant sites, such as liver, lung or bone (Weidner, N., et al., The New England Journal of Medicine, 324(1): 1-8 (1991)).
Thrombospondin-1 (TSP1, MW 450,000) is a large modular matrix protein that inhibits neovascularization in vivo (Good, et al., Proc. Natl. Acad Sci. USA., 87, 6624 (1980)). The majority of the antiangiogenic activity of TSP1 resides in the central 70-kD stalk region. Synthetic peptides were prepared which mimic sequences found in the properdin-like repeat region 1 of the central stalk region of TSP1 molecule. One of these peptides, the so-called MalII, is a 19-mer of formula Ser-Pro-Trp-Ser-Ser-Ala-Ser-Val-Thr-Ala-Gly-Asp-Gly-Val-Ile-Thr-Arg-Ile-Ar g (Tolsma, et al., J. Cell Biol., 122, 497 (1993)). The MalII peptide blocked neovascularization in vivo in the rat cornea and inhibited the migration of cultured endothelial cells with an ED.sub.50 of about 1 .mu.M.
Although several angiogenesis inhibitors are currently under development for use in treating angiogenic diseases (Gasparini, G. and Harris, A. L., J Clin Oncol 13(3): 765-782, (1995)), there are disadvantages associated with several of these compounds. For example, suramin is a potent angiogenesis inhibitor, but causes (at doses required to reach antitumor activity) severe systemic toxicity in humans. Other compounds, such as retinoids, interferons and antiestrogens are safe for human use but have only a weak anti-angiogenic effect. Still other compounds may be difficult or costly to make. Short peptides are relatively simple to make and represent a cost effective method of treating disease states in which angiogenesis plays a role and in designing targeted inhibitors of angiogenesis.